When you flush a properly functioning toilet, the contents of the tank empty into the bowl and continue down the drain, creating a suction that pulls out water. When the toilet doesn't flush completely, the problem could be a flapper malfunction, a clog in the waste line or a problem with the waste pipes or the venting. Some causes are simple to diagnose and repair, but some require a plumber.
Poorly Adjusted Flapper
The toilet tank and bowl are connected by a drain pipe, and a rubber flapper fits over the drain to keep them separated. When you push the flush handle, the flapper lifts straight up, opening the pipe and allowing all the water to drain at once. This creates the suction force necessary to drain the bowl. If the flapper chain is too tight or loose, the flapper may not lift completely and the water drains too slowly. You can rectify this problem by adjusting the length of the flapper chain.
A partial drain clog can be responsible for slow draining and will probably lead to a complete clog if you don't remove it. Instead of rushing down the waste pipe and creating a suction, water seeps by the clog, allowing the contents of the bowl to collect around the obstruction and eventually block the drain altogether. You can remove most clogs with a plunger or, if that doesn't work, with a toilet plumbing auger (closet auger). Sometimes, however, paper or debris can collect in the toilet's internal P-trap, and the only way to remove it is to reach in and pull it out.
Improperly Installed Waste Pipes
If you've noticed that your toilet has always been hard to flush, the fault may be in the waste line. The suction force depends on the diameter of the waste pipes, their slope and how they are connected. If the diameter is too large, flowing water won't seal them, and if they are sloped incorrectly, water will flow too fast or not fast enough. Moreover, if a tee or elbow is in the wrong place, it can hold up the flow just enough to cause poor draining. Waste line repairs are best left to a plumber.
After water clears the immediate vicinity of the toilet and continues down the drain, the suction it creates impedes its progress unless the waste line is vented to allow in air to replace the vacuum. The toilet vents extend upward through the roof and can get blocked by debris falling from nearby trees, ice or even a dead squirrel. A telltale symptom of poor venting is gurgling from nearby drains as water rushes through them to fill the vacuum. Clear obstacles from the vent opening, or clean the vent lines with a plumbing auger or garden hose to correct his problem.